1. Field of the Invention
The present invention relates to a superconducting magnet device and particularly to a superconducting magnet employing a refrigerator cooling system.
2. Description of the Related Art
Japanese Patent No. 3,117,173 is a background art in this technical field. This literature discloses a technique in which a permanent current switch is arranged on a cooling stage on a high temperature side thermally connected to a refrigerator, whereas a superconducting coil is arranged on a cooling stage thermally connected to a low temperature side of the refrigerator, so as to enable collection of generated heat when the permanent current switch is off.
JP-A-10-247753 is another background art. This literature discloses “comprising a unit to which superconductive device is thermally connected, a separation/connection unit thermally connected to this cooling unit, and a permanent current switch thermally connected to a part that is not connected to the cooling unit, of the separation/connection unit”.
Japanese Patent No. 3,020,140 is still another background art. This literature discloses “a structure comprising a heat transfer rod thermally connected to a permanent current switch, a drive unit which mechanically moves the heat transfer rod, a two-stage refrigerator, and cooling stages connected to a high temperature side and a low temperature side of the refrigerator, wherein the drive unit is controlled to thermally connect the heat transfer rod to the cooling stage on the high temperature side or the cooling stage on the low temperature side”.
JP-A-8-138928 is still another background art. This literature discloses a unit which mechanically disconnects thermal connection between a permanent current switch and a refrigerator, as in JP-A-10-247753.
In the methods for mechanically switching the heat transfer path to the permanent current switch, disclosed in Japanese Patent No. 3,117,173, JP-A-10-247753, Japanese Patent No. 3,020,140, and JP-A-8-138928, the heat transfer rod is made to contact the cooling stage simply by the force of the drive unit. These methods have a problem that an excessive load acts on a support rod which supports the cooling stage from the normal temperature side. There are conflicting problems that while it is difficult to thicken this support rod in view of the amount of heat penetration, a predetermined contact pressure or above is necessary in order to connect the heat transfer rod to the cooling stage. If a sufficient contact pressure cannot be achieved, refrigeration capability needs to be increased excessively, contributing to a rise in cost.